A day after a US Navy fighter jet shot down a Syrian warplane, a top Russian official called it an act of aggression that assists terrorists.

“This strike can be regarded as another act of defiance of international law by the United States,” Russian Deputy Foreign Minister Sergei Ryabkov said Monday, according to Russia’s state-run news agency Tass.

“What was it if not an act of aggression? It was also an act of assistance to those terrorists whom the United States is ostensibly...